Field of the Invention
The invention relates to a method for fabricating a bipolar transistor.
Such a method is disclosed, for example, in the commonly assigned U.S. Pat. No. 5,498,567 and in the corresponding European patent EP 0 535 350 B1). A highly n-doped connection region of a collector is produced on a p-doped substrate made of silicon. The lightly n-doped collector made of silicon is applied above the terminating region of the collector. An insulating structure is produced in the substrate, which structure comprises trenches filled with insulation material and channel stop regions which are arranged below said trenches and are highly p-doped. The insulating structure surrounds the bipolar transistor to be produced laterally within the substrate. There are produced on the substrate a first SiO2 layer, above that a polysilicon layer, above that a second SiO2 layer and above that a layer made of silicon nitride. Afterward, by masked etching, a first depression reaching as far as the first insulating layer is produced, and a second depression reaching as far as the connection region of the collector is produced. In order to produce an auxiliary layer, silicon nitride is deposited and etched back, so that lateral areas of the first depression and of the second depression remain covered by the auxiliary layer and bottoms of the depressions are uncovered. Afterward, SiO2 is etched isotropically, so that a part of the first SiO2 layer is removed. In this case, the collector is uncovered below the first depression. By means of selective epitaxy, the removed part of the first SiO2 layer is replaced by a p-doped base. Afterward, a third SiO2 layer and a second polysilicon layer are deposited. The second polysilicon layer is etched back anisotropically selectively with respect to the third SiO2 layer, thereby producing spacers. Afterward, uncovered parts of the third SiO2 layer are removed by isotropic etching selectively with respect to the spacers. Afterward, a third polysilicon layer is deposited and etched back, so that an emitter is produced in the first depression and a contact to the collector is produced in the second depression. A third depression reaching as far as the first layer made of polysilicon is produced with the aid of masked etching. Afterward, conductive material is deposited and planarized, so that a contact to the emitter is produced in the first depression, a further contact to the collector is produced in the second depression and a contact to the base is produced in the third depression.
The so-called base resistance, which is the resistance between the base and a line which is connected to the base via the contact to the base, determines, besides the transition frequency and the base-collector capacitance, important characteristic quantities of the bipolar transistor, such as its maximum oscillation frequency, its gain, its minimum noise figure, its gate delay times, etc. The base resistance is preferably small.
Resistances formed between the emitter and a line connected thereto (xe2x80x9cexternal emitter resistancexe2x80x9d) and between the collector and a line connected thereto (xe2x80x9cexternal collector resistancexe2x80x9d) are readily used in integrated circuit configurations to realize ohmic load resistances. Thus, these resistances should not be too small.
It is known to reduce a boundary resistance between polysilicon and a metal by siliconizing the polysilicon, i.e. providing it with a silicide layer.
It is accordingly an object of the invention to provide a method of fabricating a bipolar transistor and a method of fabricating an integrated circuit configuration having at least one bipolar transistor of the novel type, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a bipolar transistor in which the base resistance is lower than the external emitter resistance and to an integrated circuit configuration having such a bipolar transistor.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method of fabricating a bipolar transistor, which comprises:
producing a collector doped by a first conductivity type in a substrate of semiconductor material;
producing a first insulating layer covering the collector on the substrate;
producing a polysilicon layer doped by a second conductivity type, opposite the first conductivity type, on the first insulating layer;
producing a second insulating layer on the polysilicon layer;
forming a first depression above the collector, cutting through the second insulating layer and the polysilicon layer;
subsequently producing a first auxiliary layer and a second auxiliary layer above the first auxiliary layer, and forming the first and second auxiliary layers so thin as not to fill the first depression;
anisotropically etching the second auxiliary layer until the first auxiliary layer is uncovered;
isotropically etching the first auxiliary layer selectively with respect to the second auxiliary layer until a part of the first insulating layer is uncovered;
removing a part of the first insulating layer by isotropic etching selectively with respect to the first auxiliary layer, thereby uncovering parts of the polysilicon layer and parts of the collector;
replacing the removed part of the first insulating layer with a base by selective epitaxy of silicon in situ-doped by the second conductivity type;
subsequent to producing the base, producing a third auxiliary layer;
producing spacers in the first depression on the third auxiliary layer, by deposition and etching-back of material;
isotropically etching the third auxiliary layer selectively with respect to the spacers, and uncovering the base;
depositing polysilicon doped by the first conductivity type and, thereabove, an isolating layer, and jointly patterning to produce an emitter covered by the isolating layer, partly arranged in the first depression, adjoining the base, and partly overlapping the second insulating layer;
anisotropically etching the second insulating layer selectively with respect to the isolating layer until the polysilicon layer is uncovered;
producing a silicide layer on the polysilicon layer but not on the isolating layer;
producing a base contact on the silicide layer; and subsequent to producing the silicide layer, at least partly removing the isolating layer, and producing an emitter contact on the emitter.
In other words, the object is achieved by means of a method for fabricating a bipolar transistor in which a collector doped by a first conductivity type is produced in a substrate made of semiconductor material. A first insulating layer covering the collector is produced on the substrate. A layer made of polysilicon doped by a second conductivity type, opposite to the first conductivity type, is produced on the first insulating layer. A second insulating layer is produced on the layer made of polysilicon. A first depression is produced, which cuts through the second insulating layer and the layer made of polysilicon and is arranged above the collector. After the production of the first depression, a first auxiliary layer and, above the latter, a second auxiliary layer are produced, which are so thin that they do not fill the first depression. The second auxiliary layer is etched anisotropically until the first auxiliary layer is uncovered. The first auxiliary layer is etched isotropically selectively with respect to the second auxiliary layer until a part of the first insulating layer is uncovered. A part of the first insulating layer is removed by isotropic etching selectively with respect to the first auxiliary layer, so that parts of the layer made of polysilicon and parts of the collector are uncovered. By means of selective epitaxy of silicon in situ-doped by the second conductivity type, the removed part of the first insulating layer is replaced by a base. A third auxiliary layer is produced after the production of the base. On the first auxiliary layer, spacers are produced in the first depression by deposition and etching-back of material. The third auxiliary layer is etched isotropically selectively with respect to the spacers. The base is subsequently uncovered. Polysilicon doped by the first conductivity type and, above that, an isolating layer are deposited and jointly patterned to produce an emitter which is covered by the isolating layer, is partly arranged in the first depression, adjoins the base and partly overlaps the second insulating layer. The second insulating layer is etched anisotropically selectively with respect to the isolating layer until the layer made of polysilicon is uncovered. A silicide layer is produced on the layer made of polysilicon but not on the isolating layer. A contact of the base is produced on the silicide layer. After the production of the silicide layer, the isolating layer is at least partly removed, and a contact of the emitter is produced on the emitter.
Furthermore, the above objects are achieved by method of fabricating an integrated circuit configuration with at least one bipolar transistor produced according to the above-outlined process. The method comprises:
removing a further part of the first insulating layer during the masked etching of the first insulating layer for the purpose of uncovering the first collector contact;
by producing the polysilicon layer, replacing the removed further part of the first insulating layer by at least one part of an emitter of a further bipolar transistor whose conductivity type is opposite to the conductivity type of the bipolar transistor.
The base adjoins the layer made of polysilicon. The silicide layer is arranged between the layer made of polysilicon and the contact to the base. Consequently, the base resistance is smaller compared with a bipolar transistor without a silicide layer.
The polysilicon layer is undercut by virtue of the isotropic etching of the first insulating layer. The undercutting contributes to the overlap between the base and the collector. Since the undercutting can be precisely controlled, the overlap can be small, so that a capacitance formed by the base and the collector can be very small.
On account of the isolating layer, no silicide is formed at horizontal areas of the emitter. Since the contact to the emitter is produced on the emitter, i.e. on a horizontal area of the emitter, no silicide is arranged between the contact to the emitter and the emitter. Consequently, the resistance which is formed by the emitter and by the contact to the emitter and forms at least part of the external emitter resistance is greater than the base resistance.
A silicide can be formed at lateral, uncovered parts of the emitter. However, this does not constitute a disadvantage since the contact to the emitter does not adjoin the lateral parts of the emitter.
A horizontal cross section of the emitter is greater than a horizontal cross section of the-first depression, so that the emitter partly overlaps the second insulating layer. Consequently, a mask with a larger opening than the first depression is used for producing the emitter by patterning of the polysilicon and of the isolating layer. This is advantageous since a misalignment of the mask with regard to the first depression does not have the consequence that a horizontal area of the emitter is formed within the first depression. A silicide would be formed on such a horizontal area since it is not covered by the isolating layer, so that the contact to the emitter would adjoin silicide, which would result in a lower external emitter resistance.
The second auxiliary layer serves for enabling the patterning of the first auxiliary layer by isotropic etching. Isotropic etching is advantageous relative to anisotropic etching since the first auxiliary layer is not bombarded with ions which could pass through the first insulating layer into the substrate and could cause defects there.
The patterning of the second auxiliary layer by anisotropic etching is less critical since the ions used in this case would also have to get through the first auxiliary layer in addition to the first insulating layer in order to reach the substrate. Consequently, fewer defects are produced during the anisotropic etching of the second auxiliary layer than during patterning of the first auxiliary layer by anisotropic etching.
The same applies analogously to the third auxiliary layer and to the spacers. In this case, the first auxiliary layer corresponds to the third auxiliary layer and the second auxiliary layer corresponds to the spacers.
In order to obtain an external collector resistance which is greater than the base resistance, it is advantageous to provide the following method steps:
Before the production of the first insulating layer, a connection region of the collector is formed in the form of a buried layer which is doped by the first conductivity type, is arranged below the collector and has a higher dopant concentration than the collector. Before the production of the first insulating layer, there is produced in the substrate a first contact of the collector, which reaches as far as the connection region of the collector.
The first insulating layer is produced in such a way that it covers the first contact of the collector. After the production of the second insulating layer and before the production of the first auxiliary layer, a second depression is produced, which, in the region of the first contact of the collector, reaches as far as the first contact of the collector and, outside the region of the first contact of the collector, reaches as far as the first insulating layer and is arranged beside the first depression.
During the anisotropic etching of the second auxiliary layer, a protective mask covers the second depression. During the production of the emitter, the polysilicon and the isolating layer are patterned to produce a second contact of the collector, which is covered by the isolating layer, is arranged in the second depression and on the first contact of the collector and partly overlaps the first insulating layer. After the production of the silicide layer on the layer made of polysilicon, the isolating layer on the second contact of the collector is at least partly removed. Afterward, a third contact of the collector is produced on the second contact of the collector.
Since horizontal areas of the second contact of the collector are covered by the isolating layer, no silicide can be formed thereon. Consequently, the third contact is produced directly on the second contact of the collector, so that the external collector resistance is large compared with a bipolar transistor in which a silicide is arranged between the second contact and the third contact of the collector.
Since the second contact of the collector overlaps the first insulating layer, a horizontal cross section of the second contact of the collector is greater than a horizontal cross section of that part of the second depression which reaches as far as the first contact of the collector. Consequently, during the patterning of the polysilicon and of the isolating layer for the purpose of producing the second contact of the collector, even in the event of misalignment of a mask used in the process, it is possible to prevent a horizontal area of the second contact of the collector from being formed within the part of the second depression. A silicide would be formed on such a horizontal area since it is not covered by the isolating layer, so that the third contact of the collector would adjoin silicide, which would result in a lower external collector resistance.
The protective mask can be removed after the anisotropic etching of the second auxiliary layer and before the production of the base. On account of the protective mask, the second auxiliary layer above the second depression is not removed-during the anisotropic etching of the second auxiliary layer, so that the first contact of the collector remains protected during the production of the base.
As an alternative, no protective mask is used during the anisotropic etching of the second auxiliary layer. In exchange, a mask covering the first contact of the collector is used during the production of the base.
The first depression and the second depression can be produced simultaneously or successively.
That part of the second depression which reaches as far as the first contact of the collector can be produced by masked etching after the production of the remaining part of the second depression. Thus, in order to produce the second depression, etching is effected firstly as far as the first insulating layer with a first mask, and subsequently with a second mask as far as the first contact of the collector.
As an alternative, the second depression can be produced for example as follows:
After the production of the first insulating layer and before the production of the layer made of polysilicon, the first contact of the collector is uncovered by means of masked etching. Afterward, the layer made of polysilicon is produced, so that it adjoins the first contact of the collector. The second depression can then be produced in one step, since the layer made of polysilicon directly adjoins the first contact of the collector in the region of the first contact of the collector but adjoins the first insulating layer outside the region of the first contact of the collector.
The first insulating layer, the second insulating layer and the second auxiliary layer are preferably produced from SiO2. The first auxiliary layer is preferably produced from silicon nitride. In this case, it is advantageous to produce a protective layer made of silicon nitride on the second insulating layer. The first depression and the second depression are produced after the production of the protective layer. The protective layer is preferably removed during the removal of the first auxiliary layer.
The protective layer is attacked during the removal of the first auxiliary layer since both the protective layer and the first auxiliary layer are composed of silicon nitride and the first auxiliary layer is removed by isotropic etching.
During the anisotropic etching of the second auxiliary layer, generally the protective layer is partly uncovered. This is the case in particular when no protective mask is used in the process. However, even when using the protective mask which covers the second depression, for example, an opening of the protective mask is preferably chosen to be so large that, in the event of misalignment of the protective mask with regard to the first depression, the first depression is nevertheless uncovered. The protective layer protects, instead of the first auxiliary layer, parts of the second insulating layer during the isotropic etching of the first insulating layer.
It is also possible to use other materials for the various layers.
Preferably, after the production of the base and before the production of the third auxiliary layer, a third insulating layer is produced from SiO2, and is so thin that the first depression is not filled by the third insulating layer and by the third auxiliary layer. The third auxiliary layer is preferably produced from silicon nitride. The spacers are preferably produced from polysilicon since polysilicon can be dry-etched with very great selectivity with respect to silicon nitride. The third auxiliary layer is etched isotropically selectively with respect to the spacers until the third insulating layer is uncovered. Afterward, the third insulating layer is etched isotropically selectively with respect to the third auxiliary layer until the base is uncovered.
After the production of the silicide layer, it is possible to produce an intermediate oxide in which there are produced a first contact hole, which reaches as far as the silicide layer, a second contact hole, which reaches as far as the emitter, and a third contact hole, which reaches as far as the second contact of the collector. The contact of the base is produced in the first contact hole. The contact of the emitter is produced in the second contact hole. The third contact of the collector is produced in the third contact.
The isolating layer acts as an etching stop during the production of the contact holes of different depths in the intermediate oxide, so that the contact holes can be produced simultaneously without the emitter being abraded. Uncovered parts of the isolating layer are removed after the production of the contact holes.
The substrate is composed, for example, of silicon (Si), germanium (Ge), or SiGe.
In order to avoid scattered light during the exposure of photoresist for the purpose of producing a photoresist mask used to produce the first depression and/or the second depression, it is advantageous to deposit a layer made of amorphous silicon before the production of the photoresist mask. The layer made of amorphous silicon is removed during the etching of the layer made of polysilicon selectively with respect to silicon nitride for the purpose of producing the first depression.
Preferably, before the production of the first insulating layer, an insulating structure is produced in the substrate, which structure laterally surrounds that part of the bipolar transistor to be produced which is arranged in the substrate. If the bipolar transistor is part of an integrated circuit configuration, then the insulating structure insulates the bipolar transistor from other semiconductor components of the integrated circuit configuration which are arranged in the substrate.
The insulating structure may comprise insulation trenches filled with insulating material, or an insulation, produced by thermal oxidation, and an underlying and adjoining diffusion region. The diffusion region is doped by a second conductivity type opposite to the first conductivity type.
If a further bipolar transistor whose conductivity type is opposite to the conductivity type of the bipolar transistor is produced for the integrated circuit configuration, then preferably parts of the bipolar transistor and parts of the further bipolar transistor are produced simultaneously in order to reduce the process complexity.
By way of example, during the masked etching of the first insulating layer for the purpose of uncovering the first contact of the collector, a further part of the first insulating layer is removed in the region of the further bipolar transistor. By virtue of the production of the layer made of polysilicon, the removed further part of the first insulating layer is replaced by at least one part of an emitter of the further bipolar transistor. The bipolar transistor is an npn bipolar transistor and the further bipolar transistor is a pnp bipolar transistor. As an alternative, the bipolar transistor is a pnp bipolar transistor and the further bipolar transistor is an npn bipolar transistor.
The base has a lower dopant concentration than the polysilicon layer.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for fabricating a bipolar transistor and method for fabricating an integrated circuit configuration having such a bipolar transistor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.